Sql 将总数增加100的update语句

如何编写一份更新对账单，将今年第一个月编写的所有发票的信用总额字段添加到100

我想写一句话，比如：

 Update dbo.invoices
 set credittotal (this is the problem right here where i dont know how to     add       100 to this)
 where invoices = (the first of the year)

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加上100意味着列的新值将是当前值加上100：

UPDATE dbo.invoices
SET    credittotal = credittotal + 100
WHERE  DATEPART (dy, invoices) = 1

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set credittotal=credittotal+100

？提示：这正是您通常向变量添加100的方式。